November 1946 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
like members of the Mogollon Rim Collins Collectors Club
play an important role in the preservation of vintage radio equipment.
They own a nice example of a Collins model 30K-5 medium power
(250 W) AM transmitter
(photo at right) that is the subject
of this 1946 article in Radio News. At 5½ feet tall
and weighing 350 pounds, the 30K is many times the volume and weight
of a modern solid state transmitter (transceiver + PA) having the
same capability. Its advertised price in 1946 was $1,450, which
is equivalent to $17.7k in
2014 money - obviously not in the affordability range of your
typical amateur radio enthusiast.
* Many such websites and photos of the
K30 exist, but I chose K0CXX because they have model airplanes hanging
around the building.
New Transmitter for Amateur Radio
By W. Bruene, W0TTK, and N. Hale, W0JIH
Collins Radio Company
Introducing the salient features of a new transmitter which is
designed to fulfill the needs of a large part of the medium-power
Skillful design is combined with sturdy construction
in this Collins 30K transmitter.
The new Collins 30K has many features that are considered highly
desirable in amateur radio. A large number of the more than seventy-five
hams in the Collins organization had a voice in its specifications.
After thorough consideration of all their suggestions, the design
engineer selected the following as the features that could be included
consistent with versatility and economy: 1. 500 watts input on c.w.,
somewhat less on phone; 2. single ended output; 3. complete neutralization
on all bands; 4. low driving power; 5. band-switching; 6. v.f.o.
control, highly accurate and stable; 7. fixed bias; 8. electronic
keying; 9. minimum power supplies; 10. high efficiency; 11. high
level class "B" modulation; 12. speech clipper; 13. low pass audio
filter; 14. single control operation; 15. accessibility for operation
The result is a versatile, thoroughly engineered transmitter
with high performance and easy operation. A more detailed description
of the circuit and components follows.
1. Power input. A power input of 500 watts on
c.w. and 375 watts on phone was chosen because it is obtainable
with low exciter power requirements, moderate tube costs, and economical
power supply design. The 115 volt, 60 cycle a.c. power consumed
by a transmitter of this size does not require special wiring.
2. Single ended r.f. stage. An Eimac 4-125A
is used in the final r.f. stage. This tube is admirably suited to
features 1 and 2. Using one tube in a single ended output greatly
simplifies bandswitching, since only one end of each coil is switched.
The 4-125A, a notable advance in beam power tetrode development,
easily handles the desired power. Its high efficiency (75% to 80%)
assures maximum power output. The grid driving power required is
well below 10 watts and can be supplied from a low powered exciter
setting on the operating desk.
3. Neutralization. The 4-125A is again suited
to the purpose. It requires no neutralization, even on 10 meters.
Thus the circuit is simplified and a clean signal is transmitted
on all bands.
4. Low driving power. Discussed under the second
5. Bandswitching. The 30K has bandswitching
throughout, so that the operator can change bands quickly. This
is particularly desirable on the higher frequency bands which usually
are open for a limited number of hours each day. Five separate tank
coils are utilized (10 and 11 meters use the same coil), each with
its own variable link. The link couples the antenna or feeders to
the cold end of the plate coil. All links are mechanically coupled
together and are operated by a control knob on the front panel.
They feed directly into 52 ohm to 73 ohm transmission lines. For
higher impedance transmission lines an external tuning circuit should
be employed. Either balanced or unbalanced antennas can be used.
6. V.f.o. control. This was accomplished with
a high degree of success. The dial is calibrated directly in frequency.
The exciter unit will be discussed in detail in the latter part
of this article.
7, 8, 9. Power supplies. Economical design dictates
that the power supplies be kept to a minimum. A high voltage supply,
low voltage supply, and a bias supply provide all the d.c. power
required by the 30K. A single high voltage supply feeds both the
r.f. power amplifier and the modulator tubes. Since the 4-125A tube
requires high voltage and low current, as do the 75TH modulators,
a pair of 866A rectifiers can deliver easily the required plate
current for both stages. The low voltage supply provides voltage
for the 4-125A screen, the speech amplifier, and the modulator driver.
It also supplies bias voltage for the 4-125A and the modulator tubes.
With the 4-125A biased to cut-off, keying can be accomplished in
10. High efficiency. In the 30K circuit, plate
efficiency of the 4-125A is between 75 and 80%. The tank and coupling
circuits are designed for low loss, thus delivering maximum power
to the antenna.
11, 12, 13. Speech equipment. Speech amplifiers
and modulator equipment are on one chassis located in the transmitter
cabinet. The audio input will accommodate either crystal or high
impedance dynamic microphones. The amplifier is conventional, with
the exception of the speech clipper and filter employed.
Clipper circuit employed in transmitter.
Prior to the development of the 30K, a considerable amount of
investigation in the laboratories had proved the advantages of audio
peak clipping. In congested frequencies such as amateur bands, or
in adverse atmospheric conditions, peak clipping raises the effective
modulation level and provides greater intelligibility at the receiving
end. It also prevents over-modulation, because audio peaks and transients
are clipped before reaching the modulator.
A low pass filter, following the speech clipper, attenuates all
frequencies above 4000 c.p.s. This cut-off frequency is high enough
to preserve the naturalness of the voice, yet eliminates excessive
bandwidth. In field tests, listeners have reported that the 30K
takes a comparatively narrow band width in amateur bands.
A pair of Eimac 75TH tubes are utilized as modulators. The modulation
transformer has a separate winding for modulating the 4-125A screen.
A manually operated "phone-c.w." switch is located in the modulator
unit, and is controlled by a knob on the front of the transmitter.
For c.w. operation, this switch opens the filament circuit of the
modulator tubes and shorts the secondary of the modulation transformer.
The audio gain in the speech amplifier is controlled by a knob
on the front panel. The clipping level is adjusted manually from
the rear of the cabinet, and can be set to occur at any desired
percentage of modulation.
14. Single control operation. The transmitter plate
switch and the exciter switch are connected in series. Just by leaving
the transmitter switch in the "On" position, the exciter switch
will have complete control of transmitting and receiving functions.
The exciter switch has an extra section for use as a receiver disabling
If push-to-talk operation is desired, the "push-to-talk" switch
is connected in series with the plate control section of the exciter
switch. Both transmitter and exciter switches are left in the "transmit"
position. An extra relay is then necessary for disabling the receiver.
Calibration procedure is unchanged. Terminals are provided for operation
of a receiver disabling relay, and also for an antenna change-over
relay in the transmitter.
15. Accessibility. The cabinet is of sturdy
welded steel construction. All units, tubes, and wiring are easily
accessible through the full length rear door. Electrical interlocks
break all high voltages when the cabinet is opened. The door may
be locked with a key to prevent entrance by unauthorized persons.
All operating controls are conveniently located on the front
panel. Meters are mounted on an insulated panel located behind a
glass window near the top of the transmitter. Another window permits
observation of the 4-125A d. amplifier.
The exciter unit for the 30K can be adapted easily to drive a
variety of higher powered transmitters. All circuits are ganged
together and controlled by a single tuning knob. The variable frequency
oscillator is a peacetime application of a rugged and highly stable
wartime development. The dial is calibrated directly in frequency,
and is accurate to within 1 kc. in the 40 meter band. Accuracy on
the other bands is directly proportional. The 1 kc. allowable deviation
includes the frequency error due to all normal operating conditions.
The v.f.o. operates in the 160 meter band.
One control bandswitches all circuits simultaneously. The exciter
covers the 80, 40, 20, 15, 11, and 10 meter bands. In the "calibrate"
position, the exciter can be tuned to zero beat with a received
signal without turning on the transmitter.
Electronic keying provides fast, clean c.w. operation. When the
key is removed from the jack, the circuit is automatically closed
for phone operation. The exciter output is fed through a 73 ohm
coaxial transmission line to a link on the grid coil in the transmitter.
Operation of the exciter is extremely simple - one control switch,
a bandswitch, and single dial tuning. It can be used either as a
transmitter or as a versatile exciter. Frequency control is rapid,
accurate, and reliable.
The 30K has been given strenuous and critical tests in actual
amateur operation. The ease and reliability of operation were firmly
established. As was expected, the speech clipper was highly satisfactory,
and effectively raised the modulation level. The signal was kept
clean, and intelligibility was definitely increased. The low-pass
audio filter maintained a narrow band width. It can be said truthfully
that the clipper enabled the operator to obtain and maintain solid
contacts that would have been lost otherwise in static and crowded
Posted November 5, 2014